A communication system is a facility that enables communication between two or more entities such as user terminal equipment and/or network entities and other nodes associated with a communication system. The communication may comprise, for example, communication of voice, electronic mail (email), text messages, data, multimedia and so on.
The communication may be provided by fixed line and/or wireless communication interfaces. A feature of wireless communication systems is that they provide mobility for the users thereof. An example of a communication system providing wireless communication is a public land mobile network (PLMN). An example of the fixed line system is a public switched telephone network (PSTN).
A cellular telecommunication system is a communications system that is based on the use of radio access entities and/or wireless service areas. The access entities are typically referred to as cells. Examples of cellular telecommunication standards includes standards such as GSM (Global System for Mobile communications), GPRS (General Packet Radio Servers), EDGE (Enhanced Data Rates for GSM Evolution), AMPS (American Mobile Phone System), DAMPS (Digital AMPS), WCDMA (Wideband Code Division Multiple Access), UMTS (Universal Mobile Telecommunication System) and CDMA 2000 (Code Division Multiple Access 2000). In addition to cellular telecommunication systems, other types of wireless communication systems are also known. For example, WLAN networks (wireless local area networks) are widely known. There are several WLAN standards in use, for example one for 2.4 GHz frequency (802.11b) and one for 5 GHz frequency area (802.11a). There is also a technology called WiMax which is typically classified to be an instance of wireless LAN technology. Radio communication technology using UWB (ultra wide band) techniques are presently being developed.
Combining different wireless technologies in a portable device such as a mobile phone or, more generally, a mobile station produces a number of problems. One difficult problem is interference of different wireless systems combined in the same device. For example, GSM transmissions can disturb WLAN connections, since harmonics of some GSM transmissions may in some cases fall within a WLAN frequency band. For example, the third harmonic of transmissions using the GSM1800 technology will fall at least partially on or near the 5 GHz WLAN frequency band. This is a significant problem, when a WLAN receiver and a GSM transmitter are in close proximity, for example integrated in the same device such as a mobile phone.
Harmonics are a problem for example in a device transmitting on a frequency band near 850 MHz, and having a Bluetooth receiver. Certain cellular technologies such as the GSM850, CDMA850, and WCDMA850 use a frequency band near or around 850 MHz. The third harmonic frequency of transmissions of such cellular transmitters fall at least partially on or near the Bluetooth reception frequency band.
Filtering can help remove the unwanted harmonics, but the filtering structures can be complicated and expensive. Also, filtering in high frequency such as in the 5 GHz WLAN frequency band, it is technically difficult to build high quality filters. This is due for example to self-resonance effects resulting from parasitic capacitances of inductors. Inductors can also be implemented using RF lines or micro lines on a substrate. However, the substrate material also causes loss of RF signal. The 5 GHz WLAN band is troublesome for filtering also because of its wide bandwidth of 675 MHz, which complicates filter design.
In some types of mobile terminals transmitted and received signals are separated by filtering in a so-called duplexer. Such a duplexer can reduce interference between a transmitter of one wireless system and a receiver of another wireless system. However, some systems such as a typical GSM/EDGE mobile station use a switch to connect the transmitter and the receiver to the antenna. In such a case, adding a filter to reduce interference of the transmitter to a second service would increase complexity and cost of the receiver. Receiver and transmitter insertion losses will also increase when extra filtering is added. This results in higher current consumption in transmitter and lower sensitivity in receiver.
Filtering is very difficult also in cases where a frequency band of a cellular communications network is very close to a frequency band of a second service. The bands may be so close that sufficiently good filters cannot be reliably or economically manufactured.
In addition to harmonics, another source of disturbances is wideband noise of a transmission, which can cause a significant increase in noise level experienced by a receiver in the same device as the transmitter. For example, wide band noise of GSM1800 transmissions outside of the transmission band may disturb GPS reception at GPS L1 band (1575.42 MHz) during the transmission slot. Also, wide band noise of a GSM900 transmitter can interfere with reception on the GPS L5 band (1176.25 MHz).
A mobile terminal may locate geographically itself with multiple methods. In a cellular network a straightforward way to locate the terminal is to use the cell identification information, which indicates the identity of the current base station. A network can also locate a terminal by observing the arrival times of the transmissions from multiple base stations. A growing trend for producing location information is integration of a satellite based locator device such as a GPS or Galileo receiver in a mobile station. However, transmissions to a wireless network from a mobile station can appear as interference to a GPS receiver in the mobile station.
There may be also devices, which have shared functionalities in different physical units. Such devices are often called multipart products. The first unit of the multipart product may comprise an earpiece and a microphone, while a second unit of the multipart product communicates with the cellular networks. Units of a multipart product can communicate between each other e.g. via Bluetooth air-interface.
Some radio communications whose reception in a mobile station can suffer due to transmissions of a cellular part in the mobile station are for example digital video broadcasts (DVB), WLAN transmissions, UWB transmissions, and bluetooth transmissions. The degree to which different services may be interfered by cellular transmissions depend naturally on location of frequency bands of these services, which varies from country to country, and even between operators in a given country. For example in US one cellular band can be shared between two operators.
Terrestrial digital video broadcasting (DVB-T) was first adopted as a standard in 1997, and has been deployed throughout many areas of the world. DVB-T offers about 24 megabits per second (Mb/s) data transfer capability to a fixed receiver, and about 12 Mb/s to receivers in mobile stations. Mobile stations including such mobile receivers have already been produced and are able to receive the DVB-T signals.
While DVB-T allows high quality video broadcasting to be delivered to various devices, the DVB-T standard has certain problems with respect to mobile stations. One such problem is power usage, as mobile stations that implement DVB-T tend to consume too much power. Since mobile stations are battery powered unless plugged into a secondary power source, power usage is a critical design element. In response to this power usage and other effects of DVB-T, the DVB-H (a version of DVB for handheld devices) standard was created. DVB-H. Offers, among other things, reduced power usage as compared to DVB-T.
Because of the benefits of DVB-H over DVB-T, DVB-H is beginning to make inroads into the mobile station market. At the time of writing of this patent application, pilot projects using DVB-H technology to bring television like services to mobile devices have been started. While DVB-H is an improvement over DVB-T, DVB-H also causes certain problems. For instance, a mobile station typically will contain at least one transmitter that transmits using one or more frequency bands. The DVB-H receiver typically receives in a frequency band that is different than the one or more frequency bands used by any transmitter in the mobile station. For instance, certain mobile stations can support the global system for mobile communications (GSM) standard, and the frequency bands used by a GSM transmitter are different that the frequency band used by a DVB-H receiver. Nonetheless, transmitting using one frequency band can still cause interference in the frequency band used by the DVB-H receiver in the same device.
In the European Union DVB-H version, the reception band is on the lower side of the GSM frequency band (GSM900, which has a transmission frequency band of 880-915 MHz) and the reception frequency band is far enough that transmissions on the 900 MHz frequency band are not band-blocking the DVB-H receiver. Nonetheless, the wide band noise may be a problem also in European DVB-H reception.
In terms of the U.S. standards, transmissions on the 850 MHz frequency band may generate a second harmonic, which will be at least partially on top of the U.S. DVB-H reception frequency band of 1670-1675 MHz. Also, the 850 MHz frequency band transmission will generate wide band noise to the U.S. DVB-H frequency band. The most difficult frequencies are 835.0-837.5 MHz in transmission, since these frequencies generate harmonics directly on top of the U.S. DVB-H frequency band.
There will shortly be also other than DVB-H based digital TV services available in US. One known system in development is known as MediaFlo, which is a digital TV service driven by Qualcomm Inc. The centre frequency of transmission in the MediaFlo system is 719 MHz. This digital TV system will use OFDM based modulation method, which is similar than used in DVB-H system. Thus similar interoperability problems can be assumed with MediaFlo system than with DVB-H system. The wideband noise from 850 MHz band cellular system may generate interfering noise to the MediaFlo reception channel.